System and process for reducing chemical losses in the treatment of ash produced in a recovery boiler of a wood pulping process

Abstract

The present invention relates to a process for treating ash recovered from a recovery boiler. The process is designed to recover valuable pulping chemicals, such as potassium, sodium, carbonate, and sulfate. The ash treatment process produces a chloride-rich purge stream containing sulfate, carbonate, potassium, and sodium, which can be beneficially used in the wood pulping process. To separate the chloride from the beneficial chemicals, the chloride-rich purge stream is sent to an anion separation unit, such as a nanofiltration unit or an ion exchange unit. This effectively separates or removes the chloride from the purge stream, allowing the beneficial chemicals to be recycled and used in the wood pulping process.

Description

[Technical Field]

[0001] Wood pulping processes typically include systems and processes for treating ash produced when concentrated black liquor is burned in a recovery boiler. Ash treatment systems and processes are typically designed to recover pulping chemicals, such as carbonates and sulfates, that are useful in the pulp mill. Chlorine (Cl), present in the pulp mill in the form of chlorides, adversely affects the recovery of these chemicals. One way to address chlorides is to remove chloride-rich streams from the process. See U.S. Pat. No. 8,721,837 (the '837 patent) and U.S. Pat. No. 9,574,303, the disclosures of which are expressly incorporated herein.

[0002] In the '837 patent, the dissolved ash solution is first concentrated in an evaporative crystallizer and then subjected to a glaserite (3K2SO4.Na2SO4) crystallizer, which produces a glaserite slurry and a chloride-rich purge stream that leaves the process. The problem is that the chloride-rich purge stream contains chemicals useful in the wood pulping process (sodium, carbonate, and sulfate), as well as potassium that can be recovered as potassium sulfate (also known as SOP), effectively wasting these useful chemicals. Summary of the Invention

[0003] In wood pulping processes, ash is recovered from recovery boilers. As previously mentioned, the ash contains large amounts of valuable pulping chemicals, such as sodium, carbonate, and sulfate. In the process of processing the ash to recover sodium sulfate and burkeite, and in some cases to produce potassium sulfate, a chloride-rich purge stream is typically produced. This chloride-rich purge stream contains large amounts of sulfate, carbonate, and sodium that can be beneficially used in the wood pulping process. The present invention relates to a process for treating this chloride-rich purge stream to recover sulfate, carbonate, potassium, and sodium. Essentially, this process effectively removes chlorides from the chloride-rich purge stream, leaving a treated stream containing these valuable chemicals that can be used in the wood pulping process.

[0004] In one embodiment, the present invention involves a method for treating ash produced in a wood pulping process, comprising the steps of: a.Recovering ash from recovery boilers in wood pulping processes; b. mixing the ash with an aqueous solution in an ash dissolving tank and dissolving the ash in the aqueous solution to form a dissolved ash solution containing sodium, chloride, potassium, carbonate, and sulfate; c. concentrating the dissolved ash solution by producing sodium salts which are recovered in a recovery process, and in the process of concentrating the dissolved ash solution, producing a glaserite slurry and a chloride-rich purge stream containing sodium, potassium, carbonates, and sulfates; d.Recovering potassium sulfate (SOP) from glaserite slurry; e. recovering sodium, potassium, sulfate, and carbonate from the chloride-rich purge stream by treating the chloride-rich purge stream in an anion separation unit to produce a treated stream that is chloride-reduced and contains sodium, potassium, carbonate, and sulfate; and f. Recycling the treated stream containing sodium, potassium, carbonates, and sulfates and small amounts of chlorides to the ash dissolving tank and mixing the treated stream with the ash in the ash dissolving tank.

[0005] Another embodiment involves a slightly different process for treating ash produced in a wood pulping process. In this embodiment, the method comprises: a.Recovering ash from recovery boilers in wood pulping processes; b. Sending the ash to the ash leaching tank; c. Mixing the ash with water or an aqueous solution to leach chlorides from the ash to obtain a slurry of sodium sulfate and burkeite crystals and a solution containing sodium, potassium, chloride, sulfate, and carbonate; d. subjecting the slurry of sodium sulfate and burkeite crystals and the solution containing chloride, sodium, potassium, sulfate, and carbonate to a solid-liquid separation device to produce a slurry or wet cake and a chloride-rich, sodium, potassium, sulfate, and carbonate-containing separation liquid; e. Returning the slurry or wet cake to the wood pulping process; f. recovering sodium, potassium, carbonate, and sulfate from the chloride-rich centrate by subjecting the centrate to a nanofiltration process or an ion exchange process to produce a chloride-rich purge stream and a chloride-depleted recycle stream containing sodium, potassium, carbonate, and sulfate; and g. Recycling the recycled stream, which is reduced in chloride but contains sodium, potassium, carbonate, and sulfate salts, to the ash leaching tank and mixing the recycled stream with the ash and aqueous solution; Includes:

[0006] Other objects and advantages of the present invention will become clearly apparent from a consideration of the following description and accompanying drawings which are merely illustrative of the invention. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of a method or process for treating ash from a recovery boiler used in a wood pulping process.

[0008] [Figure 2] FIG. 2 is a schematic diagram of an alternative process for treating ash from a recovery boiler used in a wood pulping process. DETAILED DESCRIPTION OF THE INVENTION

[0009] Figure 1 illustrates a process for treating ash generated in a recovery boiler in a wood pulping process. Before describing the overall process in detail, one objective of the present invention is to provide an ash treatment process that facilitates the treatment of ash and the significant recovery of valuable chemicals that can be recycled and used throughout the wood pulping process. Because ash contains chlorides, the process of the present invention removes a significant amount of chlorides from the process. However, the chloride purge stream contains valuable wood pulping chemicals that can be used in other parts of the wood pulping process or recovered as valuable fertilizer (SOP). Therefore, the process effectively separates chlorides from the chloride-rich purge stream, leaving a treated stream containing valuable chemicals that the present invention aims to recover and reuse. As described below, in one embodiment, the process effectively separates chlorides in the purge stream from the valuable chemicals targeted for recovery using an anion separation unit, such as a nanofiltration unit or ion exchange unit. As used herein, the term "anion separation unit" refers to a device or unit effective for separating chlorides in a liquid solution from other ions and compounds in the liquid solution. This is illustrated in the processes shown in Figures 1 and 2.

[0010] With particular reference to FIG. 1, ash from the recovery boiler is sent to a dissolving tank 20, where it is dissolved in water. In some cases, all or substantially all of the ash from the recovery boiler is sent to tank 20. In other cases, only a portion of the ash from the recovery boiler is sent to tank 20. The ash sent to tank 20 is dissolved to form a dissolved ash solution. The dissolved ash solution is sent to an evaporative crystallizer 22, which concentrates the dissolved ash solution to form a concentrated dissolved ash solution (purge stream) and a concentrate containing a mixture of sodium sulfate and burkeite (2NaSO4.Na2SO3) crystals. The concentrated dissolved ash solution is typically relatively rich in chloride and potassium. The concentrate produced by evaporative crystallizer 22 is sent to a solid-liquid separator 24, where the burkeite and sodium sulfate crystals are separated from the mother liquor. The burkeite and sodium sulfate are typically returned to the wood pulping operation. The mother liquor produced by solid-liquid separator 24 is returned to evaporative crystallizer 22 via line 26.

[0011] As shown in FIG. 1, the purge stream in the form of a concentrated ash solution is sent to glaserite crystallizer 28 through line 25. Upon entering crystallizer 28, the concentrated ash solution is cooled, preferably adiabatically. Adiabatic cooling refers to lowering the temperature of a system without removing heat from the system. One common method of adiabatic cooling is to reduce the pressure within the crystallizer. Because temperature and pressure in a closed system are directly proportional, a decrease in one reduces the other. In one embodiment, the adiabatic cooling process in glaserite crystallizer 28 occurs until the temperature of crystallizer 28 reaches approximately 50°C. The adiabatic cooling process crystallizes glaserite in crystallizer 28, forming a glaserite slurry that is sent from crystallizer 28 to solid-liquid separator 30. During the adiabatic cooling of the concentrated ash solution in stream 25, crystallizer 28 produces purge stream 32. Purge stream 32 contains a relatively high concentration of chlorides. In addition to high concentrations of chlorides, purge stream 32 may contain valuable chemicals such as sodium, potassium, sulfates, and carbonates that can be used in various parts of the wood pulping process.

[0012] Before describing further processing of the chloride-rich purge stream 32, we will discuss the processing of the glaserite slurry produced by the glaserite crystallizer 28. The glaserite slurry produced in the adiabatic cooling crystallizer 28 is sent to the solid-liquid separator 30. Various types of solid-liquid separators, such as filters or centrifuges, can be used. In either case, the solid-liquid separator 30 separates the glaserite crystals from the glaserite slurry. The glaserite crystals are then sent to the SOP crystallizer 38. The function of the SOP crystallizer 38 is to recover SOP from the glaserite crystals. To accomplish this, water is sent to the SOP crystallizer 38. The water can be cooled to a temperature ranging from 0°C to 25°C. Here, the glaserite crystals come into contact with the water. When the glaserite crystals come into contact with the water in the SOP crystallizer 38, the sodium sulfate and potassium sulfate within the glaserite crystals dissolve in the water. After dissolution, a portion of the potassium sulfate recrystallizes as SOP within the SOP crystallizer. Therefore, when enough water is added to dissolve the sodium sulfate, the excess SOP remains as crystals. What remains in the SOP crystallizer 38 is sodium sulfate and an SOP solution containing recrystallized SOP.

[0013] The recrystallized SOP is recovered through a process in which the sodium sulfate-rich solution is sent to solid-liquid separator 40 to separate the recrystallized SOP from the sodium sulfate-rich solution. As shown in FIG. 1, the sodium sulfate solution may contain potassium sulfate. This solution may be recycled to glaserite crystallizer 28 via line 42.

[0014] As discussed above, the glaserite crystallizer 28 produces a chloride-rich purge stream 32 containing valuable chemicals that can be used in the wood pulping process. One objective of the present invention is to provide an effective and efficient process for recovering these valuable chemicals from the chloride-rich purge line 32. To achieve this objective, as shown in FIG. 1, the chloride-rich purge stream is sent to a so-called anion separation unit. The chloride-rich purge stream may be cooled before being sent to the anion separation unit. Additionally, water is typically added to the chloride-rich purge stream. The anion separation unit is effective at removing chlorides from the purge stream and effectively separating useful chemicals, such as sodium, carbonate, and sulfate, from the chlorides for recycling back into the process of FIG. 1. A variety of anion separation units can be used. As shown in FIG. 1, two options include a nanofiltration unit or an ion exchange unit. In either case, the chloride-rich stream 32 is sent to either a nanofiltration unit or an ion exchange unit, both of which are effective at removing chlorides. In the case of a nanofiltration unit, a chloride-rich purge stream is sent to the nanofiltration unit, which produces a permeate and a retentate. While sodium and potassium, equivalent to chloride, pass through the nanofiltration unit, the valuable chemicals, namely, sodium, potassium, carbonate, and sulfate, are rejected and become part of a stream called treated stream 32A. Treated stream 32A now contains valuable chemicals, such as potassium, sodium, carbonate, and sulfate. The permeate from the nanofiltration unit is then heavily enriched in chloride. It should be noted that in some cases, it may be desirable to adjust the pH of the chloride-rich purge stream for processing in the nanofiltration unit. Typically, the pH adjustment is lowered to a range of about 9 to 11.5, preferably to a range of about 10 to 11. Sulfuric acid or sesquisulfate from the bleach plant can be used to lower the pH.

[0015] Essentially the same results can be achieved in an ion exchange unit. That is, chlorides in the chloride purge stream 32 are removed by appropriately selecting an ion exchange having an anion exchange resin that selectively removes chlorides. In this case, the chloride purge stream 32 is sent to and passed through the ion exchange, where the resin collects the chlorides. The treated effluent from the ion exchange contains pulping chemicals that can be recycled via line 32A back to the ash leach tank 20. At various points, it becomes necessary to regenerate the resin in the ion exchange unit that collected the chlorides. Therefore, a regeneration fluid is sent through the ion exchange to collect the chlorides and generate a regeneration waste stream that is removed from the process. The regeneration waste stream can be further treated or appropriately discharged.

[0016] Turning now to the embodiment of Figure 2, this also illustrates a process for treating ash generated from the combustion of black liquor in a recovery boiler. Ash recovered from the recovery boiler is sent to an ash leaching tank, where water or an aqueous solution is mixed with the ash. Chlorides and potassium are leached from the ash, producing a slurry of sodium sulfate and burkeite crystals and a solution containing sodium, potassium, chloride, sulfate, and carbonate. The slurry containing sodium sulfate and burkeite crystals, along with the solution containing sodium, potassium, chloride, sulfate, and carbonate, is then sent to a solid-liquid separator. In the example shown in Figure 2, the solid-liquid separator is a centrifuge. The centrifuge produces a slurry or wet cake that is recycled to the wood pulping process. Additionally, the centrifuge produces a liquor containing valuable chemicals, namely, sodium, sulfate, and carbonate.

[0017] The centrate produced by the centrifuge is mixed with water downstream of the centrifuge. After mixing with water, the centrifuge is directed to a nanofiltration unit or an ion exchange unit, both of which function as described above with respect to the embodiment shown in FIG. 1 . In the case of a nanofiltration unit, the chloride-rich centrate is sent to the nanofiltration unit, which produces a permeate and a retentate. Because sodium and potassium, in amounts equivalent to chloride, pass through the membrane of the nanofiltration unit, the permeate constitutes a chloride-rich purge stream, which can be discharged from the process or subjected to further processing. The retentate produced by the nanofiltration unit may contain valuable pulping chemicals, such as carbonates and sulfates, and is recycled back to the ash leaching tank 20. Substituting an ion exchange unit for the nanofiltration unit would function and operate in the same manner as described above.

[0018] In addition to recovering valuable pulping chemicals, many other benefits are derived from this invention. By treating the chloride-rich purge stream as described above, the water load required to dilute the salt to the appropriate concentration is reduced to a level small enough that it does not increase the evaporation load in the evaporator. This is because the retentate produced by the nanofiltration unit or the treated brine recovered from the ion exchange unit produces a much smaller stream than if the treatment were performed upstream of the evaporative crystallizer 22. Placing a nanofiltration or ion exchange unit downstream of the glaserite crystallizer 28 reduces potassium losses and produces a chloride-rich purge stream at a temperature closer to that desired for the nanofiltration and ion exchange units. Furthermore, this process significantly reduces potassium loss in the final purge stream, thereby significantly improving SOP productivity. Placing an anion separation device on the purge stream rather than upstream of the evaporative crystallizer 22 significantly reduces the amount of water required for dilution. The above process also allows for the addition of potassium chloride (KCl) to the SOP crystallizer 38. By removing chlorides without removing significant amounts of sulfates and carbonates, it becomes economically feasible to add potassium chloride to produce additional SOP. Pulp mills typically have excess sulfate, so this process effectively uses the excess sulfate to convert potassium chloride into higher-value SOP.

[0019] The present invention may, of course, be practiced in other specific ways than those specifically described herein without departing from its scope and essential characteristics. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes that come within the meaning and range of equivalency of the appended claims are intended to be embraced therein.

Claims

1. 1. A method for treating ash produced in a wood pulping process, comprising: recovering ash from a recovery boiler of said wood pulping process; mixing the ash with an aqueous solution in an ash dissolving tank to dissolve the ash in the aqueous solution to form a dissolved ash solution containing sodium, chloride, potassium, carbonate, and sulfate; concentrating said dissolved ash solution, and in the process of concentrating said dissolved ash solution, producing a glaserite slurry and a chloride-rich purge stream containing said sodium, potassium, carbonate, and sulfate salts; recovering potassium sulfate (SOP) from said glaserite slurry; recovering said potassium, sodium, sulfate, and carbonate from said chloride-rich stream by treating said chloride-rich stream in an anion separation unit to produce a chloride-reduced treated stream comprising said sodium, potassium, carbonate, and sulfate; and recycling the treated stream containing the sodium, potassium, carbonate, and sulfate salts to the ash dissolving tank and mixing it with the ash in the ash dissolving tank.

2. The method of claim 1 , wherein the anion separation unit comprises a nanofiltration unit or an ion exchange unit.

3. 10. The method of claim 1, comprising adding water to the chloride-rich stream prior to treating the chloride-rich stream in the anion separation unit.

4. separating glaserite crystals from said glaserite slurry; conveying said glaserite crystals and water to a sulfate of potassium (SOP) crystallizer to contact said glaserite crystals with said water; dissolving the glaserite crystals in the water in the SOP crystallizer to obtain a solution containing sodium sulfate and the SOP; recrystallizing a portion of the SOP in the potassium sulfate crystallizer to obtain a sodium sulfate-rich solution in the SOP crystallizer; and 10. The method of claim 1, further comprising passing the sodium sulfate rich solution containing the recrystallized SOP to a solid-liquid separator to separate the recrystallized SOP from the sodium sulfate rich solution.

5. 3. The method of claim 2, wherein the anion separation unit is a nanofiltration unit, and the method comprises maintaining the chloride-rich stream at a pH of about 9 to 11.5 prior to entering the nanofiltration unit.

6. 1. A method for treating ash produced in a wood pulping process, comprising: a. Recovering ash from a recovery boiler in a wood pulping process; b) conveying the ash to an ash leaching tank; c) mixing the ash with water or an aqueous solution in the ash leach tank to leach chlorides from the ash to obtain a slurry containing sodium sulfate and burkeite crystals and a solution containing sodium, potassium, chloride, sulfate, and carbonate salts; d) subjecting the slurry containing the sodium sulfate and burkeite crystals and the solution containing chlorides, sulfates, and carbonates to a solid-liquid separation device to produce a separated liquid containing sodium, potassium, sulfates, and chlorides, and a concentrate containing a slurry or wet cake; e. recycling the slurry or wet cake to the wood pulping process; f) recovering sodium, potassium, carbonate, and sulfate from the chloride-rich permeate by subjecting the permeate to a nanofiltration or ion exchange process to produce a chloride-rich purge stream and a chloride-depleted recycle stream containing sodium, carbonate, and sulfate; g. recycling said recycle stream, which is reduced in chlorides but contains sodium, potassium, carbonates, and sulfates, to said ash leach tank and mixing said recycle stream with said ash and said water or aqueous solution.

7. 7. The method of claim 6, wherein the solid-liquid separation device comprises a centrifuge, the centrifuge producing the separation liquid and the slurry or wet cake.

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